Two unit zoom lens system

ABSTRACT

The present invention provides a two-unit zoom lens system comprising, in order from the object side, a first lens unit G1 having a negative refracting power and a second lens unit G2 consisting of a first positive sub-lens-unit P1 , a first negative sub-lens-unit N1, a second positive sub-lens-unit P2 and a second negative sub-lens-unit N2 and having a positive refracting power as a whole, and including an air gap between said first and second lens units, which is variable for zooming. In this zoom lens system which is made waterproof, inexpensive and of high performance, the amount of movement of the first lens unit G1 in association with zooming is reduced.

BACKGROUND OF THE INVENTION

The present invention relates generally to a taking zoom lens system forlens shutter cameras, which comprises two units, the first unit having anegative refracting power and the second unit having a positiverefracting power, and more particularly to a two-unit zoom lens systemdesigned for waterproof purposes.

So far, many positive-unit preceding type zoom lens systems, representedby a two-unit or positive/negative-unit zoom lens system and athree-unit or positive/positive/negative-unit zoom lens system, havebeen proposed and practically used as zoom lens systems for lens shuttercameras. However, the positive-unit preceding type zoom lens systems areunpreferred, because the amount of movement of the first units inassociation with zooming is so increased that the mechanical structurefor water is complicated.

In order to achieve waterproofness with a simple structure, it is thusdesired to eliminate, or at least reduce, the amount of movement of thefirst unit in association with zooming. Negative-unit preceding typezoom lens systems have been known to have such a feature. In particular,a two-unit or negative/positive-unit type zoom lens system is oftenpractically used as the standard zoom lens for a single-lens reflexcamera's interchangeable lens. However, the single-lens reflex cameralens is not suitable for lens shutter cameras without making somemodification thereto, because the back focus is long and hence the totallens length is increased.

As known so far in the art, reducing the total lens length may beachieved by constructing the unit on the image-taking side inpositive/negative telephoto type. For instance, Japanese ProvisionalPatent Publication No. 62-50718 sets forth an example of applying thisconcept to a two-unit or negative/positive-unit zoom lens system.

According to Japanese Provisional Patent Publication No. 62-50718,reducing the total lens length is achieved by constructing the secondunit having a positive refracting power of positive and negative lenscomponents.

However, the zoom lens system set forth in Japanese Provisional PatentPublication No. 62-50718 has a cost problem, because it comprises asmany as 9 lens components plus two additional aspherical surfaces.

SUMMARY OF THE INVENTION

In view of the disadvantages of the prior art mentioned above, a mainobject of this invention is to provide a waterproof type of a two-unitor negative/positive-unit zoom lens system in which the lens arrangementis designed to reduce the number of lenses used, or when the number oflenses used is not reduced, cost reductions are achieved by eliminatingthe use of an aspherical surface.

The two-unit zoom lens system of this invention comprises, in order fromthe object side, a first lens unit having a negative refracting powerand a second lens unit consisting of a first positive sub-lens-unit, afirst negative sub-lens-unit, a second positive sub-lens-unit and asecond negative sub-lens-unit and having a positive refracting power asa whole, and includes an air gap between said first and second lensunits, which is variable for zooming.

Preferably, the zoom lens system includes a stop between the first lensunit and the second lens unit.

Preferably, the zoom lens system conforms to the following condition(1):

    0.8<|f.sub.1 |/f.sub.W <1.6.             (1)

Here

f₁ is the focal length of said first lens unit, and

f_(W) is the focal length of the total system at the wide position.

Alternatively, it is desired that the zoom lens system conform to thefollowing condition (2):

    0.4<e.sub.T.f.sub.W /f.sub.T <3.                           (2)

Here

e_(T) is the air gap between said first and second lens units,

f_(T) is the focal length of the total system at the tele position, and

f_(W) is the focal length of the total system at the wide position.

Still alternatively, it is desired that the zoom lens system conform tothe following condition (3):

    0.3<|f.sub.N |/f.sub.2 <3.               (3)

Here

f₂ is the focal length of said second lens unit, and

f_(N) is the focal length of said second negative sub-lens-unit in saidsecond lens unit.

Still alternatively, it is desired that the zoom lens system conform tothe following condition (4):

    0.4<|r.sub.a |/f.sub.2 <3.               (4)

Here

f₂ is the focal length of said second lens unit, and

r_(a) is the radius of curvature on the object side of said firstnegative sub-lens-unit in said second lens unit.

In the ensuing description, why the lens arrangement mentioned above isused and how it acts will be explained.

Formula (1) mentioned above defines the condition for achievingwaterproofness that is the object of this invention, and relates to theamount of movement of the first unit in association with zooming. In thecase of a general negative/positive two-unit zoom lens system, theamount of movement of the first unit is minimized, when theimage-formation magnification is 1 at the intermediate focal lengthgiven by (f_(W).f_(T))^(1/2). The farther the lens system departs fromthis condition, the more the amount of movement of the first unit; inother words, it is desired that the condition (1) be satisfied. Inparticular, exceeding the upper limit of Formula (1) is unpreferred,because both the total lens length and the lens diameter of the firstunit are increased. Below the lower limit of Formula (1), on the otherhand, the power of the first unit becomes too strong, making correctionof the curvature of field and distortion difficult. This is contrary tocost reductions, because it is required to increase the number of lensesin the first unit.

Formula (2) mentioned above concerns the inter-unit space at the teleposition. When the upper limit of Formula (2) is exceeded, the totallens length is increased, failing to make the zoom lens system compact.An air gap smaller than the lower limit is unpreferred, becausemechanical interference is likely to occur by production or regulationerrors.

Formula (3) is directed to the second negative lens component in thesecond unit. In this invention, the negative lens component is locatedon the image side of the triplet arrangement of the second unitbasically comprising the positive, negative and positive lenscomponents, whereby the second unit is constructed in telephoto type,and the principal point location is brought close to the object side,whereby the back focus is reduced, resulting in achievement of areduction of the total lens length. Thus, Formula (3) is important forkeeping the zoom lens system compact. When the upper limit of Formula(3) is exceeded, the power becomes feeble and the effect of thetelephoto type becomes slender, failing to make the zoom lens systemcompact. When the lower limit is not reached, the power becomes strongenough to make the zoom lens system compact, but the curvature of fieldand distortion are likely to increase, failing to attain sufficientperformance.

Formula (4) pertains to the first negative lens component in the secondunit, defining the condition for achieving correction of sphericalaberration and coma. Departing from this conditional formula fails toattain good enough performance in terms of spherical aberration andcoma.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

The invention accordingly comprises the features of construction,combinations of elements, and arrangement of parts which will beexemplified in the construction hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-5 represent in section the lens arrangements of Examples 1-5 ofthis invention, and

FIGS. 6(a)-10(l) are aberration diagrams showing the sphericalaberrations, astigmatisms, distortions and chromatic aberrations ofmagnification of Examples 1-5 at the (a) wide, (b) standard and (c) telepositions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In what follows, some examples of the zoom lens system according to thisinvention will be explained more specifically.

The zoom lens systems according to Examples 1-5, the lens data of whichwill be given later, have a focal length of 36-68 mm and a zoom ratio of2, and the sectional views of their lens arrangements are shown in FIGS.1 to 5.

Both Examples 1 and 2 are all directed to using spherical surfaces. InExample 3 or 4 one aspherical surface is incorporated in the second unitG2, and in Example 5 two aspherical surfaces are used, one in the firstunit G1 and another in the second unit G2.

In Example 1, 3 or 5 the first unit G1 comprises, in order from theobject side, a negative meniscus lens convex on the object side and apositive meniscus lens convex on the object side, two lenses in all. InExample 2 or 4, on the other hand, the first unit G1 comprises, in orderfrom the object side, two negative lenses and a positive meniscus lensconvex on the object side, three lenses in all.

In Example 1, 2 or 4 the first positive lens component P1 of the secondunit G2 consists of a cemented positive lens and a positive lens, and inExample 3 or 5 it consists of one positive lens. In Example 1 or 4 thefirst negative lens component N1 of the second unit G2 consists of onedouble-concave lens, and in Example 2, 3 or 5 it consists of a cementedlens of negative and positive lenses.

In each example, the second positive lens component P2 of the secondunit G2 consists of one positive lens and the second negative lenscomponent N2 of the second unit G2 consists of one negative meniscuslens convex on the image side.

It is noted that if the stop is located in the second unit G2, as is thecase with Example 4, then the performance of the zoom lens system dropsconsiderably due to the eccentric arrangement of the lenses with thestop interposed between them. Thus, the stop should preferably belocated proximately to the object side of the second unit G2.Preferably, the aspherical surface in the second unit G2 should be ofsuch configuration that its positive refracting power decreasesgradually from the optical axis toward the periphery. On the other hand,it is desired that the aspherical surface in the first unit G1 be ofsuch configuration that its negative refracting power decreasesgradually from the optical axis toward the periphery.

It is noted that the symbols used hereinafter but not referred tohereinbefore mean:

f: focal length of the total system

F_(NO) : F-number

ω: half field angle

f_(B) : back focus

r₁, r₂, . . . : radii of curvature of the lens surfaces

d₁, d₂, . . . : separation between adjacent lenses surfaces

n_(d1), n_(d2), . . . : d-line refractive indices of the lenses

ν_(d1), ν_(d2), . . . : Abbe's number of the lenses

Let x denote the optical axis direction and y denote the directionnormal to the optical axis, then aspherical configuration is expressedby the following formula:

    x=(y.sup.2 /r)/[1+{1-(y/r).sup.2 }.sup.1/2 ]+A.sub.4 Y.sup.4 +A.sub.6 Y.sup.6 +A.sub.8 Y.sup.8 +A.sub.10 Y.sup.10,

provided that r is the radius of curvature on the optical axis and A₄,A₆, A₈ and A₁₀ are aspherical coefficients.

EXAMPLE 1

    ______________________________________                                                 f = 36 ˜ 49.5 ˜ 68                                                F.sub.NO =  4.78 ˜ 5.72 ˜ 7.00                                    ω = 30.96 ˜ 23.57 ˜ 17.62°                           f.sub.B = 25.01 ˜ 32.64 ˜ 43.09                          ______________________________________                                        r.sub.1 = 226.5480                                                                       d.sub.1 = 1.5000                                                                           n.sub.d1 = 1.77250                                                                       ν.sub.d1 = 49.66                        r.sub.2 = 19.4300                                                                        d.sub.2 = 4.7400                                                   r.sub.3 = 22.8680                                                                        d.sub.3 = 3.0000                                                                           n.sub.d2 = 1.78472                                                                       ν.sub.d2 = 25.68                        r.sub.4 = 37.8800                                                                        d.sub.4 = (Variable)                                               r.sub.5 = ∞ (Stop)                                                                 d.sub.5 = 1.0000                                                   r.sub.6 = 43.0970                                                                        d.sub.6 = 2.8200                                                                           n.sub.d3 = 1.48749                                                                       ν.sub.d3 = 70.20                        r.sub.7 = -21.9050                                                                       d.sub.7 = 1.5000                                                                           n.sub.d4 = 1.84666                                                                       ν.sub.d4 = 23.78                        r.sub.8 = -32.9020                                                                       d.sub. 8 = 0.2000                                                  r.sub.9 = 12.2760                                                                        d.sub.9 = 3.5600                                                                           n.sub.d5 = 1.51633                                                                       ν.sub.d5 = 64.15                        r.sub.10 = -129.6920                                                                     d.sub.10 = 1.5800                                                  r.sub.11 = -33.2070                                                                      d.sub.11 = 1.7500                                                                          n.sub.d6 = 1.83400                                                                       ν.sub.d6 = 37.16                        r.sub.12 = 22.5110                                                                       d.sub.12 = 3.0100                                                  r.sub.13 = 54.5950                                                                       d.sub.13 = 4.6700                                                                          n.sub.d7 = 1.61293                                                                       ν.sub.d7 = 37.00                        r.sub.14 = -18.8410                                                                      d.sub.14 = 7.6000                                                  r.sub.15 = -9.5260                                                                       d.sub.15 = 1.5000                                                                          n.sub.d8 = 1.77250                                                                       ν.sub.d8 = 49.66                        r.sub.16 = -23.2240                                                           ______________________________________                                        Zooming Speces                                                                f          36          49.5        68                                         d.sub.4    22.093      11.605       3.996                                     ______________________________________                                    

EXAMPLE 2

    ______________________________________                                                 f = 36 ˜ 49.5 ˜ 68                                                F.sub.NO =  4.53 ˜ 5.57 ˜ 7.00                                    ω = 30.96 ˜ 23.57 ˜ 17.62°                           f.sub.B = 25.42 ˜ 34.24 ˜ 46.34                          ______________________________________                                        r.sub.1 = 480.2200                                                                       d.sub.1 = 1.5900                                                                          n.sub.d1 = 1.83400                                                                        ν.sub.d1 = 37.16                        r.sub.2 = 17.4530                                                                        d.sub.2 = 2.6400                                                   r.sub.3 = 89.6030                                                                        d.sub.3 = 1.5100                                                                          n.sub.d2 = 1.72916                                                                        ν.sub.d2 = 54.68                        r.sub.4 = 42.1160                                                                        d.sub.4 = 1.1300                                                   r.sub.5 = 24.1140                                                                        d.sub.5 = 2.5600                                                                          n.sub.d3 = 1.84666                                                                        ν.sub.d3 = 23.78                        r.sub.6 = 72.5900                                                                        d.sub.6 = (Variable)                                               r.sub.7 = ∞ (Stop)                                                                 d.sub.7 = 1.0000                                                   r.sub.8 = 36.9230                                                                        d.sub.8 = 3.5200                                                                          n.sub.d4 = 1.51821                                                                        ν.sub.d4  = 65.04                       r.sub.9 = -17.8300                                                                       d.sub.9 = 1.5000                                                                          n.sub.d5 = 1.83400                                                                        ν.sub.d5 = 37.16                        r.sub.10 = -34.2070                                                                      d.sub.10 = 0.2000                                                  r.sub.11 = 12.4100                                                                       d.sub.11 = 3.3400                                                                         n.sub.d6 = 1.49700                                                                        ν.sub.d6 = 81.61                        r.sub.12 = 63.5900                                                                       d.sub.12 = 1.8800                                                  r.sub.13 = -35.0500                                                                      d.sub.13 = 1.5000                                                                         n.sub.d7 = 1.83400                                                                        ν.sub.d7 = 37.16                        r.sub.14 = 15.4990                                                                       d.sub.14 = 3.7500                                                                         n.sub.d8 = 1.54814                                                                        ν.sub.d8 = 45.78                        r.sub.15 = -25.8120                                                                      d.sub.15 = 1.5800                                                  r.sub.16 = 76.6070                                                                       d.sub.16 = 2.5000                                                                         n.sub.d9 = 1.57501                                                                        ν.sub.d9 =  41.49                       r.sub.17 = -29.5090                                                                      d.sub.17 = 8.1000                                                  r.sub.18 = -9.5250                                                                       d.sub.18 = 1.5100                                                                         n.sub.d10 = 1.72916                                                                       ν .sub.d10 = 54.68                      r.sub.19 = -32.6020                                                           ______________________________________                                        Zooming Speces                                                                f          36          49.5        68                                         d.sub.6    13.518       7.429       3.012                                     ______________________________________                                    

EXAMPLE 3

    ______________________________________                                                 f = 36 ˜ 49.5 ˜ 68                                                F.sub.NO =  4.80 ˜ 5.73 ˜ 7.00                                    ω = 30.96 ˜ 23.57 ˜ 17.62°                           f.sub.B = 25.89 ˜ 33.49 ˜ 43.90                          ______________________________________                                        r.sub.1 = 106.2150                                                                       d.sub.1 = 1.5000                                                                           n.sub.d1 = 1.77250                                                                       ν.sub.d1 = 49.66                        r.sub.2 = 18.3350                                                                        d.sub.2 = 4.7900                                                   r.sub.3 = 20.2790                                                                        d.sub.3 = 2.9000                                                                           n.sub.d2 = 1.80518                                                                       ν.sub.d2 = 25.43                        r.sub.4 = 29.1350                                                                        d.sub.4 = (Variable)                                               r.sub.5 = ∞ (Stop)                                                                 d.sub.5 = 1.0000                                                   r.sub.6 = 11.8750                                                                        d.sub.6 = 4.3400                                                                           n.sub.d3 = 1.48749                                                                       ν.sub.d3 = 70.20                           (Aspheric)                                                                 r.sub.7 = -38.0310                                                                       d.sub.7 = 1.6600                                                   r.sub.8 = -22.1390                                                                       d.sub.8 = 1.2000                                                                           n.sub.d4 = ν.sub.d4 = 25.43                        r.sub.9 = 60.4060                                                                        d.sub.9 = 2.1100                                                                           n.sub.d5 = 1.56732                                                                       ν.sub.d5 = 42.83                        r.sub.10 = -28.0740                                                                      d.sub.10 = 3.5400                                                  r.sub.11 = -236.4090                                                                     d.sub.11 = 2.4400                                                                          n.sub.d6 = 1.84666                                                                       ν.sub.d6 = 23.78                        r.sub.12 = -37.4090                                                                      d.sub.12 = 7.6300                                                  r.sub.13 = -8.8330                                                                       d.sub.13 = 1.5000                                                                          n.sub.d7 = 1.72916                                                                       ν.sub.d7 = 54.68                        r.sub.14 = -18.6620                                                           ______________________________________                                        Zooming Speces                                                                f          36          49.5        68                                         d.sub.4    22.210      11.658       4.002                                     ______________________________________                                                 Aspherical Coefficients                                                       6th surface                                                                   A.sub.4 = -0.36867 × 10.sup.-4                                          A.sub.6 = 0.22551 × 10.sup.-7                                           A.sub.8 = -0.59906 × 10.sup.-8                                          A.sub.10 = 0.48085 × 10.sup.-10                                ______________________________________                                    

EXAMPLE 4

    ______________________________________                                                 f = 36 ˜ 49.5 ˜ 68                                                F.sub.NO =  4.41 ˜ 5.50 ˜ 7.00                                    ω = 30.96 ˜ 23.57 ˜ 17.62°                           f.sub.B = 23.41 ˜ 32.19 ˜ 44.21                          ______________________________________                                        r.sub.1 = -4088.5440                                                                     d.sub.1 = 1.5400                                                                           n.sub.d1 = 1.77250                                                                       ν.sub.d1 = 49.66                        r.sub.2 = 18.4040                                                                        d.sub.2 = 3.0100                                                   r.sub.3 = 43.7950                                                                        d.sub.3 = 1.5000                                                                           n.sub.d2 = 1.72916                                                                       ν.sub.d2 = 54.68                        r.sub.4 = 25.3480                                                                        d.sub.4 = 1.6000                                                   r.sub.5 = 21.9920                                                                        d.sub.5 = 2.5000                                                                           n.sub.d3 = 1.80518                                                                       ν.sub.d3 = 25.43                        r.sub.6 = 53.2760                                                                        d.sub.6 = (Variable)                                               r.sub.7 = 18.9950                                                                        d.sub.7 = 4.7400                                                                           n.sub.d4 = 1.51633                                                                       ν.sub.d4 = 64.15                        r.sub.8 = -19.3720                                                                       d.sub.8  = 1.5000                                                                          n.sub.d5 = 1.80518                                                                       ν.sub.d5 = 25.43                        r.sub.9 = -68.2490                                                                       d.sub.9 = 0.7000                                                   r.sub.10 = ∞ (Stop)                                                                d.sub.10 = 0.7000                                                  r.sub.11 = 14.2830                                                                       d.sub.11 = 3.9600                                                                          n.sub.d6 = 1.52310                                                                       ν.sub.d6 = 50.84                        r.sub.12 = -37.3330                                                                      d.sub.12 = 0.8600                                                  r.sub.13 = -32.0230                                                                      d.sub.13 = 2.4900                                                                          n.sub.d7 = 1.83400                                                                       ν.sub.d7 = 37.16                        r.sub.14 = 26.2630                                                                       d.sub.14 = 2.6700                                                  r.sub.15 = 26.4790                                                                       d.sub.15 = 3.0300                                                                          n.sub.d8 = 1.60342                                                                       ν.sub.d8 = 38.01                        r.sub.16 = -34.0970                                                                      d.sub.16 = 8.1000                                                     (Aspheric)                                                                 r.sub.17 = -9.3770                                                                       d.sub.17 = 1.5600                                                                          n.sub.d9 = 1.72916                                                                       ν.sub.d9 = 54.68                        r.sub.18 = -23.8080                                                           ______________________________________                                        Zooming Speces                                                                f           36         49.5        68                                         d.sub.5    11.429       5.970       2.010                                     ______________________________________                                                 Aspherical Coefficients                                                       16th surface                                                                  A.sub.4 = 0.33367 × 10.sup.-4                                           A.sub.6 = 0.67370 × 10.sup.-7                                           A.sub.8 = -0.12555 × 10.sup.-8                                          A.sub.10 = -0.57413 × 10.sup.-11                               ______________________________________                                    

EXAMPLE 5

    ______________________________________                                                 f = 36 ˜ 49.5 ˜ 68                                                F.sub.NO =  4.82 ˜ 5.74 ˜ 7.00                                    ω = 30.96 ˜ 23.57 ˜ 17.62°                           f.sub.B = 27.52 ˜ 35.47 ˜ 46.36                          ______________________________________                                        r.sub.1 = 93.0670                                                                        d.sub.1 = 1.5000                                                                           n.sub.d1 = 1.77250                                                                       ν.sub.d1 = 49.66                        r.sub.2 = 17.3450                                                                        d.sub.2 = 4.8000                                                      (Aspheric)                                                                 r.sub.3 = 20.2580                                                                        d.sub.3 = 2.9000                                                                           n.sub.d2 = 1.80518                                                                       ν.sub.d2 = 25.43                        r.sub.4 = 29.8960                                                                        d.sub.4 = (Variable)                                               r.sub.5 = ∞ (Stop)                                                                 d.sub.5 = 1.0000                                                   r.sub.6 = 12.0320                                                                        d.sub.6 = 4.3300                                                                           n.sub.d3 = 1.48749                                                                       ν.sub.d3 = 70.20                           (Aspheric)                                                                 r.sub.7 = -40.7490                                                                       d.sub.7 = 1.6200                                                   r.sub.8 = -22.4920                                                                       d.sub.8 = 1.2000                                                                           n.sub.d4 = 1.80518                                                                       ν.sub.d4 = 25.43                        r.sub.9 = 59.8850                                                                        d.sub.9 = 2.1400                                                                           n.sub.d5 = 1.56444                                                                       ν.sub.d5 = 43.78                        r.sub.10 = -28.1010                                                                      d.sub.10 = 3.6000                                                  r.sub.11 = -210.7850                                                                     d.sub.11 = 2.4900                                                                          n.sub.d6 = 1.84666                                                                       ν.sub.d6 = 23.78                        r.sub.12 = -36.9810                                                                      d.sub.12 = 7.6400                                                  r.sub.13 = -8.9400                                                                       d.sub.13 = 1.5000                                                                          n.sub.d7 = 1.72916                                                                       ν.sub.d7 = 54.68                        r.sub.14 = -17.4830                                                           ______________________________________                                        Zooming Speces                                                                f          36          49.5        68                                         d.sub.4    22.241      11.670       4.001                                     ______________________________________                                                 Aspherical Coefficients                                                       second surface                                                                A.sub.4 = -0.25258 × 10.sup.-5                                          A.sub.6 = 0.95586 × 10.sup.-8                                           A.sub.8 = -0.14887 × 10.sup.-9                                          A.sub.10 = 0                                                                  6th surface                                                                   A.sub.4 = -0.31976 ×  10.sup.-4                                         A.sub.6 = -0.47847 × 10.sup.-7                                          A.sub.8 = -0.32988 × 10.sup.-8                                          A.sub.10 = 0.25768 × 10.sup.-10                                ______________________________________                                    

The spherical aberrations, astigmatisms, distortions and chromaticaberrations of magnification of Examples 1-5 at the (a) wide, (b)standard and (c) tele positions are shown in the aberration diagrams ofFIGS. 6-10.

Set out below are the values of the conditional formulae (1)-(4) inExamples 1-5.

    ______________________________________                                        Example   Example   Example  Example Example                                  1         2         3        4       5                                        ______________________________________                                        (1)  1.37     0.97      1.38   0.92    1.35                                   (2)  2.12     1.59      2.12   1.07    2.12                                   (3)  0.78     0.83      0.88   1.03    0.95                                   (4)  1.19     1.53      0.79   1.48    0.78                                   ______________________________________                                    

As explained above, the present invention provides a two-unit zoom lenssystem in which the amount of movement of the first unit in associationwith zooming is reduced and which is made waterproof, inexpensive and ofhigh performance.

What we claim is:
 1. A two-unit zoom lens system comprising, in orderfrom the object side:a first lens unit having negative refracting power,and a second lens unit consisting of, in order from the object side, afirst positive sub-lens unit, a first negative sub-lens unit locatedwith a first separation between said first negative sub-lens unit andsaid first positive sub-lens unit, a second positive sub-lens unit witha second separation between said second positive sub-lens unit and saidfirst negative sub-lens unit and a second negative sub-lens unit with athird separation between said second negative sub-lens unit and saidsecond positive sub-lens unit, said second lens unit having a positivepower as a whole, wherein for zooming from a wide angle end to atelephoto end, a separation between said first lens unit and said secondlens unit is varied while the first, second, and third separations arekept constant.
 2. A two-unit zoom lens system as claimed in claim 1,wherein a stop is disposed between said first lens unit and said secondlens unit.
 3. A two-unit lens system as claimed in claim 1, whichsatisfied the following condition (1):

    0.8<|f.sub.1 |/f.sub.w <1.6              (1)

where f₁ is the focal length of said first lens unit, and f_(w) is thefocal length of the total system at the wide angle position.
 4. Atwo-unit zoom lens system as claimed in claim 1, which satisfies thefollowing condition (2):
 0. 4<e_(T).f_(w) /f_(T) <3 (2)where e_(T) isthe air gap between said first and second lens units, f_(T) is the focallength of the total system at the tele angle position, and f_(W) is thefocal length of the total system at the wide angle end.
 5. A two-unitlens system as claim in claim 1, which satisfies the following condition(3):

    0.3<|f.sub.N |/f.sub.2 <3                (3)

where f₂ is the focal length of said second lens unit, and f_(N) is thefocal length of said second negative sub-lens unit in said second lensunit.
 6. A two-unit lens system as claimed in claim 1, which satisfiesthe following condition (4):

    0.4<|r.sub.a |/f.sub.2 <3                (4)

where f₂ is the focal length of said second lens unit, and r_(a) is theradius of curvature on the object side of said first negative sub-lensunit in said second lens unit.
 7. A two-unit zoom lens systemcomprising, in order from the object side,a first lens unit havingnegative refracting power, and a second lens unit consisting of, inorder from the object side, a first positive sub-lens unit, a firstnegative sub-lens unit located with a first separation between saidfirst negative sub-lens unit and said first positive sub-lens unit, asecond positive sub-lens unit with a second separation between saidsecond positive sub-lens unit and said first negative sub-lens unit anda second negative sub-lens unit with a third separation between saidsecond negative sub-lens unit and said second positive sub-lens unit,and having positive power as a whole, wherein for zooming from a wideangle end to a telephoto end, said second lens unit is moved as a unitalong the optical axis, thereby varying a separation between said firstlens unit and said second lens unit.
 8. A two-unit zoom lens system asclaimed in claim 7, wherein a stop is disposed between said first lensunit and said second lens unit.
 9. A two-unit lens system as claimed inclaim 7, which satisfies the following condition (1):

    0.8<|f.sub.1 |/f.sub.W <1.6              (1)

where f₁ is the focal length of said first lens unit, and f_(W) is thefocal length of the total system at the wide angle position.
 10. Atwo-unit zoom lens system as claimed in claim 7, which satisfies thefollowing condition (2):

    0.4<e.sub.T.f.sub.W /f.sub.T <3                            (2)

where e_(T) is the air gap between said first and second lens units,f_(T) is the focal length of the total system at the tele angleposition, and f_(W) is the focal length of the total system at the wideangle end.
 11. A two-unit lens system as claim in claim 7, whichsatisfies the following condition (3):

    0.3<|f.sub.N |/f.sub.2 <3                (3)

where f₂ is the focal length of said second lens unit, and f_(N) is thefocal length of said second negative sub-lens unit in said second lensunit.
 12. A two-unit lens system as claimed in claim 7, which satisfiesthe following condition (4):

    0.4<|f.sub.a |/f.sub.2 <3                (4)

where f₂ is the focal length of said second lens unit, and r_(a) is theradius of curvature on the object side of said first negative sub-lensunit in said second lens unit.